1. Field of the Invention
The present invention relates generally to optical waveguides, and more particularly to optical waveguides fabricated in glass-ceramic materials.
2. Technical Background
The term “glass” is most commonly defined as an amorphous, inorganic material which solidifies from a molten state without crystallization, and may therefore be better characterized as a supercooled liquid having an irregular atomic structure rather than a solid. Glasses are most frequently fabricated by fusing selected silicates with certain oxides. In contrast, a “glass-ceramic” material possesses both a glass (or vitreous) phase and a crystallite phase within a unitary structure. The glass phase forms a matrix having a characteristic porosity, within which the crystallite phase is dispersed. There are a variety of methods for fabricating glass-ceramic materials. Glass and glass-ceramic materials are generally both regarded as dielectrics, meaning they are non-conductive, have very low electrical conductivity, or transmit electric force or electromagnetic radiation via a process other than electrical conduction.
The term “optical waveguide” has a variety of different definitions, each somewhat dependent upon the particular technical embodiment, practical application, or field of use being considered. One area of particular interest relates to optical telecommunications, or the transmission of information or data via light waves propagating through waveguides such as optical fibers. Another area of interest relates to photonics, or the development of devices which produce, amplify, guide, or control the properties of light to achieve some useful purpose, such as optical amplifiers, lasers, switches, biosensors, and myriad other devices. It is readily apparent that many photonic devices are designed for and used in optical telecommunications, but may similarly be implemented in a variety of other scientific and industrial applications as well. The transmission of light in specific infrared or near-infrared bands is of greatest interest for optical telecommunications, but other applications in the infrared, visible, and ultraviolet spectra are meaningful in different photonics applications. As a general rule, waveguides support or control the propagation of light using variations in the refractive index of materials within the waveguide (such as between adjacent materials, layers, interfaces, boundaries, or along gradients). As used herein, the term “optical waveguide” is therefore intended to mean structures fabricated from one or more dielectric materials defining variations in refractive index which support and/or control the propagation of light, most notably in the infrared, visible, and/or ultraviolet spectra.
Certain characteristics of glass-ceramic materials make them particular interesting to researchers in the telecommunications and photonics fields. For example, the ability to “dope” or impregnate selected portions of a waveguide with optically-active materials (such as erbium or other rare-earth constituents) allows them to be used to produce or amplify light, thus creating the basis for devices such as fiber lasers, optical amplifiers, or wavelength converters.
The structure of glass-ceramic materials is particularly suited to such applications, because the porosity of the glass phase and resulting homogeneity of the glass matrix surrounding the crystallite phase can be employed to control the size and distribution of crystalline particles that would otherwise produce undesirable scattering or attenuation of light within the waveguide. Furthermore, glass-ceramics may be fabricated in which the coefficient of thermal expansion (CTE) of the glass phase is the opposite of that for the crystallite phase, resulting in a glass-ceramic material having very high dimensional and thermal stability and low internal stresses through a range of temperature variations. For example, a low-thermal-expansion ceramic providing a crystallite phase having a negative CTE can be combined with a glass phase having a positive CTE to yield a thermally-stable glass-ceramic material which exhibits a negligible overall CTE within a prescribed range of operating temperatures.
However, glass-ceramic materials do present identifiable limitations for use in fabricating optical waveguides. Most notably, many conventional processes used to fabricate waveguides on a large scale require that an intermediate structure be elevated to temperatures near or above the ceramming temperatures used for conventional glass-ceramic materials. For example, drawing a glass perform into an optical fiber requires heating the perform above its softening point. Similarly, overcladding a planar waveguide may require both applying soot glass using an outside vapor deposition (OVD) or modified chemical vapor deposition (MCVD) process, and then consolidating the glass structure, both steps being performed at a relatively high temperature. Processes such as drawing a glass perform are inconsistent with glass-ceramic materials, as internal discontinuities within the glass-ceramic structure may weaken the tensile strength of the drawn glass-ceramic fiber and result in fractures and breaks. Moreover, heating the glass-ceramic to temperatures at or near the ceramming temperature may cause undesirable alterations in the porosity of the glass matrix, the composition of the crystalline phase, or fluctuations in the particle size, distribution, or homogeneity of the crystalline structures.